Lost Wind:
Target 1 face-up Special Summoned monster on the field; negate its effects, also its original ATK is halved.
No.103 (Ragnazero):
target 1 face-up Attack Position monster your opponent controls, whose current ATK is different from its original ATK;
“Lost Wind” halves the original ATK of its target, not its current ATK, hence why No.103 could not target Magnaliger in this case.